1. Field of the Invention
The present invention relates to a reflectarray.
2. Description of the Related Art
In mobile communications, if there is an obstacle such as a building on a route of a radio wave, a reception level deteriorates. For addressing this problem, there is a technique in which a reflector is provided on a high place the height of which is similar to that of the building in order to transmit a reflected wave to places where a radio wave is hard to reach. If an incident angle of the radio wave in a vertical plane is relatively small when reflecting the radio wave using the reflector, it becomes difficult for the reflector to direct the radio wave to a desired direction. The reason is that, generally, the incident angle and the reflection angle of the radio wave are the same.
For addressing this problem, it can be considered to incline the reflector such that the reflector looks into the ground. Accordingly, the incident angle and the reflection angle with respect to the reflector can be increased so that an incoming wave can be directed to a desired direction. However, from the viewpoint of safety, it is not desirable to mount the reflector by inclining it toward the ground side, since the reflector is placed on the high place similar to the building that may obstruct radio waves. From this viewpoint, it is desired to realize a reflector that can direct a reflected radio wave to a desired direction even when the incident angle of the radio wave is relatively small.
As such a reflector, an application of a reflectarray is reported (for example, refer to non-patent documents 1 and 2).
The reflectarray can be designed by arranging phase shifts of reflected waves such that a beam is directed to a desired direction. As shown in FIGS. 1A and 1B, various techniques are introduced such as a method for using a stub, a method for varying sizes and the like (for example, refer to non-patent document 3).
Non-patent document 1: L. Li et al., “Microstrip reflectarray using crossed-dipole with frequency selective surface of loops,” ISAP2008, TP-C05, 1645278.
Non-patent document 2: T. Maruyama, T. Furuno, and S. Uebayashi, “Experiment and analysis of reflect beam direction control using a reflector having periodic tapered mushroom-like structure,” ISAP2008, MO-IS1, 1644929, p. 9.
Non-patent document 3: J. Huang and J. A. Encinar, Reflectarray antennas. Piscataway, N.J. Hoboken: IEEE Press; Wiley-Interscience, 2008.
However, according to the conventional method of using a stub shown in FIG. 1A, a loss caused by the stub and unnecessary radiation from the stub may become a problem. Also, according to the method of varying the patch dimensions as shown in FIG. 1B, there is a problem in that the size of the patch is varied for producing phase shift. Therefore, there is a problem in that patches of different sizes not only change the phase shift but also exert an influence upon radiation. In addition, in these methods, there is a problem in that a range of variation of reflection phase is less than 360 degrees.
FIG. 2 shows an example of a conventional reflectarray.
In the reflectarray 1, microstrip antennas are used as array elements 10 and a metal flat plate is used as a ground plane 20. FIG. 2 shows an example in which the array element 10 is a square. The dimensions a and b of the array element 10 are determined based on a phase shift.
In order to realize a reflectarray for directing a radio wave to a desired direction by using many elements, it is necessary to arrange elements for providing a phase (reflection phase) of a predetermined reflection coefficient. Ideally, it is desirable that the reflection phase covers a range larger than 2π radian (2π radian=360 degrees) with respect to a predetermined range of a structure parameter such as the patch size.
However, in the case when the array element is configured by the microstrip antenna, there is a problem in that the phase of the reflection coefficient in a given frequency does not cover a wide range.